Add new grammar for vector search

[jonathanhogg]

One thing that probably would be worth adding new grammar for is a vector search operation, a la:

item in vector – return true or false as to whether item is contained within vector

For consistency this should accept an n-vector for item and should look for whether all of those items appear consecutively in vector.

Originally posted by @jonathanhogg in #65 (comment)

[jonathanhogg]

This will probably want an appropriate search method added to the Vector() class.